Gas column density maps of the numerical simulation at 0.01 Myr (top), 0.12 Myr (centre), and 0.30 Myr (bottom). The top panel also shows the projected positions of the stars more massive than 16 M_⊙ (black points) and those less massive than 16 M_⊙ (white points). The bottom panel shows the projected positions of the sink particles as white points. The arrows point out the 50 M_⊙ (left) and 10 M_⊙ (right) cloudlets. The centre and bottom panels show the position of the shell as a black dotted circle.